Presently, as a next-generation communication standard of LTE (Long Term Evolution), 3GPP (3rd Generation Partnership Project) is promoting standardization of LTE-Advanced. In a LTE-Advanced system, a carrier aggregation (CA) technique is introduced to achieve a higher throughput than a LTE system while ensuring backward compatibility with the LTE system. In the carrier aggregation technique, a LTE carrier (also referred to as a component carrier) having the maximum bandwidth of 20 MHz supported by the LTE system is utilized as a basic component, and it is designed to implement communications in a wider band by employing such multiple component carriers simultaneously.
In the LTE-Advanced, small cell enhancement, where a mixture of a macro cell provided by a conventional base station and a small cell covering a smaller geographical area is used, is proposed. Typically, the small cell is disposed in a traffic intensive area such as a hot spot and an indoor area which is difficult for the macro cell to cover and is used to improve communication in these areas.
In a network architecture using the small cell enhancement, utilization of inter-site CA or inter-eNB CA, where a user equipment (UE) communicates using a macro base station (macro-eNB) serving a macro cell and a small base station (small-eNB) serving a small cell simultaneously, is discussed. In typical inter-site or inter-eNB CA, the macro base station serves as an anchor base station and manages the small base station serving as a non-anchor base station to implement the inter-site or inter-eNB CA with the user equipment.
FIG. 1 is a schematic view of the inter-site CA in the small cell enhancement. As is shown in FIG. 1, in the inter-site or inter-eNB carrier aggregation in the small cell enhancement, for example, reliability required control signals or C-plane data (Signaling Radio Bearer: SRB) are communicated by a macro base station serving as an anchor base station to a user equipment via a macro cell, and wideband communication required data signals or U-plane data (Data Radio Bearer: DRB) are communicated by a small base station serving as a non-anchor base station to the user equipment via a small cell.
Meanwhile, it is specified in the LTE system and the LTE-Advanced system that a base station assigns an uplink dedicated resource to a user equipment and uses the assigned uplink dedicated resource to obtain various information such as request information and feedback information from the user equipment. Examples of the uplink dedicated resources include a scheduling request, a PUCCH-CQI (Physical Uplink Control Channel-Channel Quality Indicator) and a SRS (Sounding Reference Signal).
The scheduling request is a resource for requesting a base station for an uplink grant (UL grant) to allow for data transmission when uplink data for transmission arise in the user equipment. The PUCCH-CQI is a resource for feeding downlink communication quality information back to the base station. The SRS is a resource for causing a base station to measure uplink communication quality information.
The uplink dedicated resources are assigned to a user equipment by a RRC (Radio Resource Control) layer signal. It is specified that upon occurrence of a predefined release trigger, the assigned uplink dedicated resources are autonomously released by a user equipment. For example, the release trigger of uplink dedicated resources may include a release command by a RRC layer signal, expiration of a TA (Time Alignment) timer, over-retransmission of the scheduling request and execution of a retransmission procedure.
It is specified that in the case where the inter-site or inter-eNB carrier aggregation is configured, a single base station (anchor base station) basically transmits and receives RRC signaling. If multiple base stations transmit and receive the RRC signaling, bearers for C-plane data or a SRB must be set between the user equipment and the respective base stations, which is not desirable from the viewpoint of network complexity.
Meanwhile, it is assumed that resources in a cell served by each base station are basically managed by that base station. This is because it is not practical that a macro base station or an anchor base station manages resources of all connected small base stations.
Thus, when an uplink dedicated resource in a cell served by a small base station is reassigned to a user equipment (for example, at restarting uplink data), a reassignment procedure of uplink dedicated resources as set forth will be performed among the user equipment, the macro base station and the small base station.
Specifically, it is assumed that after an earlier configured or assigned uplink dedicated resource in a small cell has been released, the uplink dedicated resource in the small cell must be reassigned to the user equipment due to some reasons. As is shown in FIG. 2, at step S1, a macro base station managing the small base station serving the small cell sends the small base station a resource assignment request for assigning the uplink dedicated resource to the user equipment.
Upon receiving the resource assignment request, at step S2, the small base station assigns the uplink dedicated resource to the user equipment and sends the macro base station a resource assignment response indicative of the assigned uplink dedicated resource.
Upon receiving the resource assignment response, at step S3, the macro base station sends the user equipment a RRC Connection Reconfiguration.
Upon receiving the RRC Connection Reconfiguration, at step S4, the user equipment performs a RRC connection reconfiguration operation based on the indicated uplink dedicated resource in the small base station and sends the macro base station a RRC Connection Reconfiguration Complete after completion of the operation.
Upon receiving the RRC Connection Reconfiguration Complete, at step S5, the macro base station sends the small base station an assignment completion indication. Then, radio communication is established between the user equipment and the small base station, and the user equipment can use the assigned uplink dedicated resource.
See 3GPP TS 36.321 V11.1.0 (2012-12) for further details, for example.